Many of the world's remaining gold deposits are considered to be refractory or double refractory in nature. Refractory ores are those in which the recoveries of gold by conventional cyanidation are typically noneconomic. Many gold ores are sulfide refractory, meaning the gold is inaccessible to gold lixiviants because it occurs as finely disseminated particles within sulfide mineral crystals or as a solid solution in the sulphide matrix. The cost of size reduction associated with liberating this gold is often prohibitive, and, in the case of gold occurring as a solid solution, size reduction is ineffective.
This problem has been overcome by oxidizing the sulfides contained in the ore, thereby liberating the gold from the sulfide matrix and rendering it amenable to leaching by cyanide or other lixiviants. Several methods exist to accomplish oxidation but the two most common are roasting and pressure oxidation. Pressure oxidation can be performed under alkaline conditions, as in the process disclosed by U.S. Pat. No. 4,552,589, or acid conditions, as in the process disclosed by U.S. Pat. No. 5,071,477.
Acid pressure oxidation, or autoclaving, of sulfides in refractory gold ores involves subjecting feed slurry to temperatures of approximately 190° C. to 225° C. in an oxygen atmosphere. If the refractory ore contains carbonate, it must be pre-acidulated to release the CO2 prior to treatment in the autoclave. This is necessary because generation of CO2 within the autoclave will cause increased venting of the autoclave, resulting in the loss of oxygen, and therefore an increase in oxygen consumption, and higher heating costs. Pre-acidification of the autoclave feed is generally performed to maintain the equivalent carbon dioxide levels in the ore to between about 0.1 and about 0.7% by weight.
While not wishing to be bound by any theory, roasting of refractory gold ores converts sulfide sulfur, primarily pyrite, to sulfur dioxide and hematite according to the following equation:4FeS2+11O2⇄2Fe2O3+8SO2 
Roasting of gold ores is typically performed between 550° C. and 750° C. The temperature of roasting as well as the composition and flow rate of the gaseous phase are optimized according to the mineralogical and/or chemical composition of the gold bearing feed material.
During roasting, an excess of oxygen is required to oxidize pyrite and arsenopyrite to hematite rather than magnetite. Ores containing a high level of calcium carbonate will release CO2, thereby effectively diluting the oxygen content in the gas phase of the roaster and contributing to the formation of magnetite.
In the roaster, a number of highly exothermic reactions can occur when high sulfur and/or arsenide-containing particles react with the oxygen. It is possible that temperature extremes can occur at the molecular level, which may cause fusion to eutectic mixtures of iron oxides and iron sulfides. When these fused particles occlude gold, the recovery of gold in subsequent cyanidation operations will be reduced.
Low roaster temperatures, though reducing pyrite oxidation to hematite, can improve arsenic retention through the formation of pyroarsenate (2FeO.AS2O3.). Unfortunately, lower roasting temperatures can also cause the formation of low porosity maghemite, which occludes gold particles and reduces the recovery of gold by cyanidation.
Gold can be occluded by both magnetite and maghemite that is formed during the incomplete oxidation of pyrites. Both of these minerals are magnetic, and therefore susceptible to magnetic concentration.